Antennas may be used for a variety of purposes, such as communications or navigation, and portable radio devices may include broadcast receivers, pagers, or radio location devices (“ID tags”). The cellular telephone is an example of a wireless communications device, which is nearly ubiquitous. A relatively small size, increased efficiency, and a relatively broad radiation pattern are generally desired characteristics of an antenna for a portable radio or wireless device. Additionally, as the functionality of a wireless device continues to increase, so too does the demand for a smaller wireless device which is easier and more convenient for users to carry. One challenge this poses for wireless device manufacturers is designing antennas that provide desired operating characteristics within the relatively limited amount of space available for antennas. For example, it may be desirable for an antenna to communicate over multiple frequency bands and at lower frequencies.
Newer designs and manufacturing techniques have driven electronic components to relatively small dimensions and reduced the size of many wireless communication devices and systems. Unfortunately, antennas, and in particular, broadband antennas, have not been reduced in size at a comparable level and often are one of the larger components used in a smaller communications device.
Indeed, antenna size may be based upon operating frequency or frequencies. For example, an antenna may become increasingly larger as the operating frequency decreases. Reducing the wavelength may reduce the size of the antenna, but a longer wavelengths may be desired for enhanced propagation. At high frequencies (HF), 3 to 30 MHz for example, used for long-range communications, efficient antennas, for example, transmitting antennas, may become too large to be portable, and wire antennas may be required at fixed stations. Thus, it may become increasingly important in these wireless communication applications to reduce not only the antenna size, but also to design and manufacture a reduced size antenna having the greatest gain for the smallest area over the desired frequency bands.
The instantaneous 3 dB gain bandwidth, also known as half power fixed tuned radiation bandwidth, of electrically small antennas is thought to be limited under the Chu-Harrington limit (“Physical Limitations Of Omni-Directional Antennas, L. J. Chu, Journal of Applied Physics, Vol. 19, pp 1163-1175, December 1948). One form of Chu's Limit provides that the maximum possible 3 dB gain antenna bandwidth limited to 1600(πr/λ)3 percent, where r is the radius of the smallest sphere that can enclose the antenna, and λ is the free space wavelength. This may be for single mode antennas matched into circuits. Unfortunately, such an antenna fitting inside a radius=λ/20 spherical envelope may not have more than 6.1% of this bandwidth. Further, practical antennas seldom approach the Chu's limit bandwidth. An example is a relatively small helix antenna enclosed by r=λ/20 sphere size operated at 1.2% bandwidth, e.g. ⅕ of Chu's Limit. Small antennas having increased bandwidth for size may thus be desired.
Canonical antennas include dipole and the loop antennas, in line and circle shapes. They translate and rotate electric currents to realize the divergence and curl functions, for example. Various coils may form hybrids of the dipole and the loop. Antennas may be linear, planar, or volumetric in form, e.g. they may be nearly 1, 2 or 3 dimensional. Optimal envelopes for antenna sizing may be Euclidian geometries such as a line, a circle, and a sphere, which may provide increased optimization of a relatively short distance between two points, increased area for circumference, and increased volume for decreased surface area respectively. It may be desirable to know the antennas that provide the greatest radiation bandwidth in these sizes. A broadband electrically large (r>λ/2π) antenna, for example, the spiral antenna, may provide a high pass response with theoretically unlimited bandwidth above a lower cutoff. At electrically small size, however, (r<λ/2π), the spiral may provide only a quadratic, bandpass type response with greatly limited bandwidth.
Planar antennas may be increasingly valuable for their ease of manufacture and product integration. The elementary planar dipole may be formed by radial electric currents flowing on a metal disc (“Theory Of The Circular Diffraction Antenna,” A. A. Pistolkors, Proceedings of the Institute Of Radio Engineers, January 1948, pp 56-60). Circular and linear notches for feeding may be desired. A circle of wire may give the same radiation pattern, and it may be preferred for ease of driving. Elements to extend the bandwidth of wire loop antennas may be desired. Radio wave expansion occurs at the speed of light. If the speed of light were reduced, antenna size would also be reduced.
U.S. Patent Application Publication No. 2009/0212774 to Bosshard et al. discloses an antenna arrangement for a magnetic resonance apparatus. In particular, the antenna arrangement includes at least four individually operable antenna conductor loops arranged in a matrix (i.e. rows and columns) configuration. Two antenna conductor loops adjacent in a row or column are inductively decoupled from one another, while two antenna loops diagonally adjacent to one another are capacitively decoupled from one another.
U.S. Patent Application Publication No. 2009/0009414 to Reykowsi discloses an antenna array. The antenna array includes multiple individual antennas arranged next to one another. The individual antennas are arranged within a radio-frequency closed conductor loop with capacitors inserted in each conductor loop.
U.S. Patent Application Publication No. 2010/0121180 to Biber et al. discloses a head coil to a magnetic resonance device. A number of antenna elements are carried by a supporting body. The supporting body has an end section that is shaped as a spherical cap. A butterfly antenna is mounted at the end of the section, and is annularly surrounded by at least one group antenna that overlaps the butterfly antenna. However, none of these approaches are focused on providing an antenna with multi-band frequency operation, while being small in size, and having desired gain for area.